The present invention relates to an optical demultiplexer primarily for use in wavelength division multiplex optical communications.
Conventional optical demultiplexers for use in wavelength division multiplex optical communications include an optical demultiplexer having optical fibers as input and output mediums as disclosed in Japanese laid-open patent publication No. 9-243855, an optical demultiplexer having an optical fiber as an input medium and an optical waveguide as an output medium as disclosed in Japanese laid-open patent publication No. 8-75948, and an optical demultiplexer having an optical fiber as an input medium and a photodetector array as an output medium as disclosed in Japanese laid-open patent publication No. 7-30485.
In the case where optical fibers are used as input and output mediums, since a focused beam spot tends to be deformed due to an abaxial aberration, etc. and the spacing between the optical fibers cannot be smaller than the outside diameter thereof, the light beam has to be focused off the axis of the optical fiber entrance surface. As a result, a large coupling loss is caused, and the loss suffers a large variation due to a slight positional displacement of the focused position. It is thus difficult to achieve an optical demultiplexer having a reduced loss and capable of operating stably.
In the case where an optical fiber is used as an input medium and an optical waveguide as an output medium, it is possible to considerably reduce the magnitude of a coupling loss caused by an abaxial aberration. However, because the spacing between adjacent channels is reduced, interchannel crosstalk is increased, resulting in a reduction in multiplexing performance.
Japanese laid-open patent publication No. 7-30485 discloses use of an optical fiber as an input medium and a photodetector array as an output medium, and the processing of crosstalk arising between adjacent channels with a neural network circuit connected to the photodetector array. However, this arrangement is problematic in that the module structure is complex and highly expensive. Since the disclosed optical demultiplexer does not incorporate the idea of correcting an abaxial aberration in the optical system, but is based on the design concept of processing an output signal from the simply arranged photodetector array with a logic circuit, no countermeasures are taken against a coupling loss, and the logic circuit is relied on to reduce crosstalk. Accordingly, the disclosed optical demultiplexer is poorer than aberration-corrected systems.
It is therefore an object of the present invention to solve the above problems of a coupling loss due to an abaxial aberration, etc. with an arrangement not based on optical design approaches.
Specifically, it is an object of the present invention to provide an optical demultiplexer having a photodetector array which comprises photodetectors that are shaped and arranged in special ways.
To achieve the above objects, there is provided in accordance with the present invention an optical demultiplexer having an optical fiber, a single collimator lens, a diffraction grating, and an array of photodetectors, characterized in that a light beam emitted from the optical fiber is demultiplexed by the collimator lens and the diffraction grating into light beams, and the light beams are focused by the collimator lens as beam spots deformed due to an aberration of an optical system of the optical demultiplexer onto the photodetectors, and said photodetectors are arranged to accommodate and detect all the deformed focused beam spots.
The array of photodetectors comprises a linear array of photodetectors. Particularly, the array of photodetectors comprises a linear array of as many photodetectors as the number of demultiplexed channels in a direction in which the light beam is demultiplexed by said diffraction grating, and if it is assumed that said diffraction grating has a diffraction order of m and a diffraction constant d, the center-to-center distance between adjacent ones of said photodetectors along the array of the photodetectors is represented by p, the focal length of said collimator lens is represented by f, the used wavelength is represented by xcex0, and the wavelength interval between the demultiplexed channels is represented by xcex94xcex, then the center-to-center distance p between adjacent ones of the photodetectors along the array of the photodetectors satisfies the following equation:
p=mfxcex94xcex/(d{square root over ( )}(1xe2x88x92(mxcex0/(2d)2)
and wherein the width Wy of said photodetectors along the array thereof ranges from 0.2 p to 0.9 p and the width Wx of said photodetectors in a direction perpendicular to the array thereof ranges from 1 Wy to 10 Wy.
The array of photodetectors may comprise a matrix of photodetectors. Particularly, the array of photodetectors may comprise a matrix of photodetectors in 1 rowsxc3x97k columns, and if it is assumed that the number of demultiplexed channels in the direction in which the light beam is demultiplexed by said diffraction grating is represented by n, then xcex1 is defined as xcex1=(kxc2x7cosxcex8)/n (xcex8 represents an angle formed between the direction in which the light beam is demultiplexed and scattered and the direction of the k columns), and if it is assumed that said diffraction grating has a diffraction order of m and a diffraction constant d, the center-to-center distance between adjacent ones of said photodetectors along the array of the photodetectors is represented by p, the focal length of said collimator lens is represented by f, the used wavelength is represented by xcex0, and the wavelength interval between the demultiplexed channels is represented by xcex94xcex, then the center-to-center distance p between adjacent ones of the photodetectors along the array of the photodetectors satisfies the following equation:
p=mfxcex94xcex/(xcex1d{square root over ( )}(1xe2x88x92(mxcex0/(2d)2).
In this arrangement, it is preferable that a plurality of photodetectors matching the shape of a plurality of focused spots corresponding to demultiplexed channels be combined to perform an operation equivalent to a desired large photodetector, for thereby electrically correcting an abaxial aberration. Therefore, the number of photodetectors in the array thereof should preferably be at least xcex1 times (xcex1xe2x89xa72 through 5) the number of demultiplexed channels in the direction in which the light beam is demultiplexed by said diffraction grating. Though there are no limitations on the number of photodetectors particularly in the direction perpendicular to the direction in which the channels are demultiplexed, since minute photodetectors are combined for use as a large photodetector, the number of photodetectors should preferably be the same as in the direction in which the channels are demultiplexed, or at least 2 and at most xcex1.
The optical demultiplexer according to the present invention has at least an optical fiber, a single collimator lens, a diffraction grating, and an array of photodetectors. Since the array of photodetectors is employed, the optical fiber and each one of the photodetectors do not need to be individually encased in a sealed package. As a result, the optical demultiplexer can be reduced in size and cost. Furthermore, the photodetectors have an effective light detecting surface whose width in the direction perpendicular to the array of photodetectors is greater than its width along the array of photodetectors. Therefore, an abaxial aberration caused by an optical system is not corrected by a special design incorporated in the optical system, but is coped with as much as possible by arranging the array of photodetectors in order to accommodate beam spots deformed by the aberration.
If the array of photodetectors comprises a matrix of photodetectors, then the number of photodetectors in the array thereof is at least xcex1 times (xcex1xe2x89xa72, or more preferably xcex1xe2x89xa75) the number of demultiplexed channels in the direction in which the light beam is demultiplexed and scattered, and at least 2 and at most xcex1 in the direction perpendicular to the direction in which the light beam is demultiplexed. Consequently, when an input light beam is demultiplexed into a plurality of light beams and the light beams are deformed by an abaxial aberration, deformed focused beam spots are projected onto auxiliary arrays of photodetectors which substantially accommodate all the deformed focused beam spots. The auxiliary arrays of photodetectors are realized within the matrix of photodetectors in an arbitrary configuration that can desirably match the shape of the deformed beam spots. As a result, the optical demultiplexer has a low coupling loss.